The present invention relates to a dry developer for electrophotography, and more particularly to a dry developer for electrophotography which is stable during use to variations in ambient conditions has good resistance to the offset phenomenon as discussed below in hot roll fixation and has an improved positive chargeability.
In general, electrophotography involves direct or indirect production of a toner image on an image receiving sheet either by a method in which developer particles (toner) electrically charged by friction to a polarity opposite to that of an electrostatic latent image are attracted to the latent image electrostatically (normal development) or by a method in which a toner electrically charged to the same polarity as that of a latent image is attracted to the latent image by an electric field generated between a magnetic brush and the latent image surface (reversal development).
The toner image is fixed to the image receiving sheet by heating, application of pressure, contact with solvent vapor or other similar means, to complete recording.
Of various fixing processes, a hot roll fixing process which involves direct contact of the toner image with the image receiving sheet has merits of excellent thermal efficiency, a high fixing speed and a small size of equipment. But, on the other hand, this process has a disadvantage of generating the so-called offset phenomenon in which toner particles adhere to the hot roll upon contact with the latter and re-adhere to a subsequent image receiving sheet. As a countermeasure against this phenomenon, a method of coating the roll surface with a releasing agent has been proposed, but this method requires complicated equipment and induces difficulties in maintenance. Accordingly, there is a keen demand for an offset-proof toner binder free of any releasing agent.
In addition, the role of the toner in producing the abovementioned image lies in providing a distinct polarity with respect to the electric field of the latent image and a stable charge quantity. A toner generally consists of a binder, a colorant and other additives, wherein the binder is the major constituent. Examples of the binder in general include coumarone-indene resins, terpene resins, resins based on styrene or compolymers thereof, polyester resins and epoxy resins, but almost none of the resins acquire positive polarity in charging by friction with iron powders.
To obtain a positively chargeable toner, a method of introducing amino groups into the binder resin and a method of adding a nigrosine dye or other additives as a positive polarity controlling agent are commonly known.
The former method, however, is disadvantageous in that although the positive chargeability is enhanced with an increase in the quantity of amino groups introduced, the chargeability fluctuates with variations in ambient humidity so that stable images cannot always be obtained.
The latter method, on the other hand, is disadvantageous in that the nigrosine dye is poor in compatibility with the binder resin used as the major constituent of the toner, that the concentration of the dye becomes non-uniform upon pulverization to degrade the image quality, that the nigrosine dye itself is unstable to ambient humidity because of its hydrophilic property, and in addition, is not suitable for coloring the toner because of its densely colored condition, etc.